1. Field of the Invention
The present invention relates to an image reading apparatus employing a CCD image sensor having multi-channel shift registers, an image forming apparatus such as a digital copier, a scanner, a facsimile machine, a digital multi-function machine or such provided with the image reading apparatus, an image inspecting apparatus for inspecting for a defect from an image read by means of the image reading apparatus, and an image forming system including the image inspecting apparatus, the image reading apparatus and the image forming apparatus.
2. Description of the Related Art
Recently an image reading apparatus has been proposed, which apparatus carries out line scanning of image information such as an original in a sub-scanning direction, forms the image information on a sensor surface of a photoelectric converting part (CCD) which acts as an image reading part, and reads the image information with the use of an output signal obtained from the photoelectric converting part. FIG. 1 (a) shows one example of such an image reading apparatus.
In FIG. 1 (a), an image reading apparatus 100 carries out operation of reading an original 5 by scanning the original 5 in a right direction with moving a first carriage 9 and a second carriage 12 by means of a scanner motor with lighting the original 15 by means of an original light source (lamp) 7, when the original 5 is placed on a contact glass 1. At this time, original image information of the original 5 on the contact glass 1 (original placement glass part) lit by means of the original light source 7 is formed on a CCD (photoelectric converting part) sensor surface by means of an imaging lens 13 through scanning first, second and third mirrors 8, 10 and 11, the original image information thus formed on the CCD sensor surface is read by the CCD, and is converted into digital data by an A-D converter mounted on a circuit substrate 14 together with the CCD. Thus, the original image data is digitally read. The original image information in the form of digital data is sent to an output apparatus, for example, and therewith, the image information is printed out. Alternatively, it is sent to a storage apparatus, and is stored therein. Thus, the thus-read image information is used in various ways.
When the original image is read with the carriages 9 and 12 moving to scan the original 5, the scanning first and second carriages 9 and 12 are moved with a fixed relationship in a direction of an arrow (sub-scanning direction) for scanning the original 5 in the sub-scanning direction. At this time, scanning speeds of the respective carriages 9 and 12 are set so that the scanning speed of the second carriage 12 is half the scanning speed of the first carriage 9.
In prior to actually reading image data by thus scanning the original, read data of a shading correction standard white plate 3 shown in FIG. 1 (a) is obtained, shading correction data is generated, which is then stored in a memory, the shading correction data is then used to normalize image data of the original 5, thus, a light quantity distribution variation, a CCD sensitivity variation, and an output variation and so forth are corrected and thus, the image information of the original is finally read with a high accuracy.
Further, along with a recent increase in the speed of the scanner, an increase in operation speed of the CCD has been demanded. In order to meet the demand, for the purpose of increasing the reading speed while the processing speed is left unincreased much, a CCD image sensor configured such that the CCD shift registers are divided into even-numbered pixels and odd-numbered pixels, and output is made therefrom separately, has been used recently.
In order to achieve further increase in the reading speed, an CCD image sensor having a plurality of register groups, i.e., more than four groups (i.e., more than four-series CCD shift registers), from which output is made separately, has been used recently. As such a type of the four-series CCD shift registers, a type in which outputs of the four series are made in a same direction, another type in which, from the two series outputs are taken in each of first half and second half directions, or such, may be used. FIG. 2 shows the latter type of a CCD image sensor. The image sensor shown in FIG. 2 is of a well-known type, includes photodiodes 21, shift gates 22, CCD analog shift registers 23, output buffers 24 and output terminals OS1, OS2, OS3 and OS4. From CP terminals, RS terminals, SH terminals, φ1, φ2 terminals and φ2B terminals, clamp clock signals, reset clock signals, shift control signals, transfer clock signals and driving clock signals are provided, respectively. The driving clock signals provided via the φ2B is the same as those of the φ1 and φ2 terminals, but are provided only to the output stages. Commonly, CCD output timing is determined by the timing of the driving clock signals provided from the φ2B terminals.
A voltage potential in the shift register's last stage varies due to a variation in the process condition when the CCD is produced, in the image sensor employing the plurality of CCD shift registers 23. Thereby, analog output delay time may have a variation. Such a variation in the analog output delay time may adversely affect the linearity of the analog output for when an entrance light quaintly is relatively low, and, as shown in FIGS. 3 and 4, a difference occurs in the linearity among the outputs of the respective CCD shift registers.
That is, difference occurs in the output delay time due to a variation in the φ2B potential well, as shown in FIG. 3. This difference results in the difference in the linearity where a difference occurs between a true signal and a CCD output due to a charge quantity. Thereby, the φ2B output and the CCD output do not correspond to one another in a one-to-one manner. As shown in FIG. 5, the difference in the linearity among the CCD shift register outputs cannot be corrected by means of shading correction. Accordingly, especially a level difference at a connection point between the first half and the second half from a middle tone through a high tone may result in a problematic image quality degradation on a finally obtained image, when the CCD in a type such that the outputs are made in opposite directions in the respective ones of the first half and the second half shift registers. It is noted that, in FIGS. 4 and 5, OSn denotes the first half part CCD output (i.e., OS1 and OS2 in the example of FIG. 2) while OSm denotes second half part CCD output (i.e., OS3 and OS4 in the example of FIG. 2).
In order to solve the problem due to the difference in the linearity among the respective CCD shift registers, a method has been proposed in which γ correction is carried out for each of the two series (even/odd-numbered pixel output type) of the shift registers. Further, as mentioned above, patent documents 1 and 2, described below, disclose a method in which, in a CCD of a four-series output type, γ correction is carried out for each CCD shift register.
In the art of the patent document 1, in order to remove variation between the right and left CCD characteristics due to difference in the CCD devices, to reproduce stable image tones with no tone difference between the left and right machine bodies, and also, to carry out image tone representation including no variation in the tones between the machine bodies, tone patterns generated by the machine bodies are output, correction processing calculation is carried out with the use of a difference in the image data between the left side and the right side of the four channel outputs for the respective tone data, which is obtained from reading of the tone patterns, correction data thus calculated is set in the left and right correction tables of the left and right correction circuits, γ correction is carried out based on the image data of the left side as standard data, and setting is made in a γ correction table.
According to the patent document 2, in an image sensor of a left and right separate reading system, in order to reduce a signal difference, and also, in order to make linearity characteristics of respective outputs coincident, the following components are provided: a color linear image sensor in the left and right separate reading system; a plurality of signal processing parts carrying out predetermined signal processing on the image signals output from the respective output terminals of the color linear image sensor; a white plate; a grey standard plate; a CPU which makes an image signal level from the white plate coincident with a first predetermined level, makes an image signal level from the gray stranded plate coincident with a second predetermined level, and obtains adjustment data for adjusting the image signal levels of other tones into levels obtained from interpolation between the first and second predetermined levels; and a lookup table (abbreviated by LUT, hereinafter) for adjusting the image signals output form the plurality of signal processing parts with the use of the adjustment data.
The following patent documents 1-3 disclose the related arts:
Patent document 1: Japanese Laid-open Patent Application No. 2000-188686;
Patent document 2: Japanese Laid-open Patent Application No. 2002-218186; and
Patent document 3: Japanese Laid-open Patent Application No. 2005-348351.